feat: Limit cache loading in cluster (#2985)

This PR adds an option to limit simultaneous cache loading in a cluster
to one node at a time.
Fixes #2707

src/data/LedgerCacheLoadingState.cpp

@@ -0,0 +1,61 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2026, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "data/LedgerCacheLoadingState.hpp"
+
+#include <memory>
+
+namespace data {
+
+LedgerCacheLoadingState::LedgerCacheLoadingState(LedgerCacheInterface const& cache) : cache_(cache)
+{
+}
+
+void
+LedgerCacheLoadingState::allowLoading()
+{
+    *isLoadingAllowed_ = true;
+    isLoadingAllowed_->notify_all();
+}
+
+bool
+LedgerCacheLoadingState::isLoadingAllowed() const
+{
+    return *isLoadingAllowed_;
+}
+
+void
+LedgerCacheLoadingState::waitForLoadingAllowed() const
+{
+    isLoadingAllowed_->wait(false);
+}
+
+bool
+LedgerCacheLoadingState::isCurrentlyLoading() const
+{
+    return cache_.get().isCurrentlyLoading();
+}
+
+std::unique_ptr<LedgerCacheLoadingStateInterface>
+LedgerCacheLoadingState::clone() const
+{
+    return std::make_unique<LedgerCacheLoadingState>(*this);
+}
+
+}  // namespace data